Friction stir welding is a beneficial metal bonding technique wherein materials such as aluminum 7075, various alloys of aluminum/lithium and other similar materials previously considered to be unweldable, can be bonded. Friction stir welding also forms joints that possess material properties similar to those of the parent materials. Further, this technique also provides welds with extremely low dimensional distortion.
Unlike fusion welding, in which portions of each work piece being joined are melted and then resolidified to form a new and different material structure in the area of the weld, friction stir welding is a solid state process. More specifically, bonding is accomplished by simultaneously stirring, heating and intermixing plasticized material from each work piece across the joint boundary. Friction stir welding generally employs a stepped cylindrical tool having a head pin (also known as a probe) surrounded at its root by a shoulder. The tool is rotated at a precise speed while being pressed against the work pieces at the joint with substantial force, which may up to about 12,000 pounds. Friction between the rotating pin and work pieces causes localized heating and plasticizing of the material adjacent to the pin. Heating reduces the yield stress of the material properties of the parent material. In operation, contact between the pin and the work pieces generates heat that causes the material to yield, thereby allowing the pin to plunge into the work pieces such that the shoulder of the tool will contact the surface of the work pieces. The material adjacent to the joint is then blended by translating the spinning tool along the joint at a carefully controlled rate. To optimize a weld formed by traditional friction stir welding techniques, it is also known to alter the angle between the shoulder and the surface of the work pieces. As the pin and shoulder spin and move relative to the work pieces the pin and shoulder simultaneously heat and move the work piece material from the leading edge of the tool to the trailing edge of the tool. Translation of the tool along the joint may be accomplished by moving the tool or by moving the work pieces relative thereto.
Propellant tanks for launch vehicles are commonly comprised of relatively thin cylindrical bodies with dome-shaped ends. The cylindrical portions of the propellant tanks are often additionally reinforced with integrated ribs, isogrids or other types of internal stiffening members. Internal stiffening members are machined in flat tank panels that are then bent subsequent to machining to form curved barrel panels that are interconnected by longitudinal welds to form cylindrical barrel sections. Two barrel sections are abutted to define a circumferential joint that is welded to form a portion of an elongated cylindrical propellant tank. The dome shaped ends are welded to the ends of the welded barrel sections to complete the propellant tank. Usually at least one dome includes an access opening that permits personnel to enter the completed propellant tanks to facilitate construction, permit inspections, etc.
One advantage of friction stir welding is that it is solid state, i.e., a weld technique that does not require weld rods and other materials traditionally used to fill a gap between abutting work pieces. In addition, since the work pieces are not brought to a temperature to cause melting of the parent material, cooling and the distortions associated therewith are less of a concern. The heat affected zone associated with the weld is also decreased since the work pieces do not become molten prior to fusing. Furthermore, by reducing the heat needed to fuse the metals, weld porosity is decreased, solid redistribution is decreased, solidification cracking is decreased, and liquefaction cracking is reduced. Reduction of the afore-mentioned common welding drawbacks lead to a low concentration of weld defects overall, which makes friction stir welding a very tolerant welding technique and well suited for fabricating propellant tanks.
Other advantages of friction stir welding include (i) the formation of welds with good mechanical properties, (ii) increased safety in which dangerous splattering of molten material is eliminated, (iii) cost savings in which no filler or gas is required, (iv) the method is easily automated and (v) the welds produced require less post welding machining.
One drawback with friction stir welding is associated with the considerable force that must be exerted by the stir welding tool on the work piece. More specifically, the work piece must be supported by a suitable backing member such as an anvil, or mandrel in the case of circumferential friction stir welding, to counteract the applied forces by the weld tool. Mandrels also help prevent geometrical mismatch between the work pieces being joined. Thus, when welding propellant tanks, the backing member must be located inside the tank. Without the use of a counteracting backing member, tank distortions and inadequate weld penetration can result.
Another disadvantage with friction stir welding is that the end weld is incomplete because a hole associated with the pin remains when the friction stir welding tool is removed from the joint. This hole must be subsequently plugged by traditional welding techniques or by friction stir plug welding techniques that employ a push or pull plug.
It is a related disadvantage of friction stir welding that the backing member must be removed after welding is completed. After welding of a closed vessel such as a tank, access to the interior of the welded tank is often limited. In some instances, access doors or hatches are provided that give access into the welded tank, but often these penetrations are typically small to facilitate proper closing of the tank. Accordingly, heavy duty backing mandrels must often be removed from very small openings in the tank, an operation that is very difficult and time consuming. Further, any contact between the backing tools and the tank may cause damage requiring expensive retesting and requalification of the propellant tank. Finally, working inside a tank invokes certain OSHA regulations related to the working in closed environments. For example, regulations may require the use of breathing apparatus or a flow of oxygen and a confined spaces permits may be required. These measures obviously increase the cost of performing the weld operation.
Friction stir welding is described in various patents. For example, U.S. Pat. No. 5,460,317 discloses a rotating tool that is used to create heat sufficient to fuse adjacent work pieces. For example, U.S. Pat. No. 5,813,592 to Midling et al., describes friction stir welding of plates. U.S. Pat. No. 4,063,676 to Lilly discloses circumferential friction stir welding wherein an expanding plug is mounted to a mandrel positioned within a tube or a pipe. U.S. Pat. No. 6,247,634 to Whitehouse describes friction stir welding of a tank wherein an anvil assembly is positioned within the tank to resist the force of the pin. U.S. Pat. No. 7,441,686 to Odheima et al., discloses a method of friction stir welding employing a backing member. U.S. Pat. No. 5,697,511 to Bampton teaches circumferential friction stir welding that employs a support structure. U.S. Pat. No. 6,070,784 to Holt et al., discloses a method of circumferential friction stir welding that uses internal supports. U.S. Pat. No. 6,257,479 to Liwiniski et al., discloses the method of friction stir welding of pipes that employs a backing tooling apparatus. Finally, U.S. Pat. No. 5,794,835 to Coloigan et al., discloses a traditional friction stir welding tool. One skilled in the art will appreciate upon review of the foregoing prior art that each require a backing anvil or other tools resist the force applied by the friction stir welding tool.
Thus, there is still a need in the field of welding to provide a friction stir welding tool that can be used to join cylindrical work pieces wherein a backing member is not required. The following disclosure describes an improved circumferential friction stir welding system and method that do not require a backing member and is ideal to join barrel sections of a booster rocket.